Antilock brake systems used in wheeled vehicles typically use relatively complex digital logic control modules in a hydraulic valve system for pressure operated wheel brakes. These antilock brake systems receive wheel speed signals from speed sensors at the wheels and analyze wheel speed behavior. The logic module is capable of determining the extent of wheel slip using the input wheel speed data.
An electronic control signal is delivered to a solenoid valve network in such systems to adjust wheel brake pressure. A wheel brake pressure dump valve and a wheel brake pressure apply valve respond to brake signals developed by the digital logic module as they are sequentially actuated. In this way, the brakes are intermittently applied and released to avoid continued locking of the wheels and to maintain the tire/road surface interface slip ratio at a desired value.
The effective coefficient of friction that exists at the tire/road surface interface will vary depending on the type and condition of the road surface and the type and condition of the tires. The coefficient of friction increases in direct proportion to the slip ratio before the effective friction coefficient at the tire/road interface reaches a maximum value. When the slip ratio exceeds a predetermined slip ratio value, the relationship between slip ratio and effective coefficient of friction reverses as the effective friction coefficient decreases.
The lateral stability of the vehicle due to slip at the tire/road surface interface also changes according to the slip ratio. Lateral stability is maintained at an acceptable level, however, if the controller achieves a slip ratio in the vicinity of the range of slip ratios where the coefficient of friction is at or near its maximum value.
In such prior art systems, pressure control solenoid valves basically achieve a dump of brake pressure or a hold brake pressure at a constant value or reapply brake pressure, but there is no control of the rate of pressure buildup at the brakes nor a control of the pressure level. Such systems also lack a means for obtaining feedback information that would determine brake pressures or pressure buildup rates for improved control and reduced braking distance and lateral control of the vehicle.